1. Field of the Invention
The present invention relates to trusses used in large span constructions.
2. Prior Art
Among the many types of roof structures for large-span constructions are prestressed trussed girders, one type of which was invented by the present applicants (Japanese Publication Application No. 60-175997, as illustrated in FIG. 17). The trussed girder 51 comprises an upper chord member 57, a lower chord member 58 arranged generally parallel to the upper member 57, and lattice 59 for supporting and joining the upper chord member 57 and the lower chord member 58. In the trussed girder 51, a steel cable 62 is arranged along the lower chord member 58 (or the upper chord member 57) extending from one end of the lower chord member 58 (or the upper chord member 57) to another end of the lower chord member 58 (or the upper cord member 57) so as to exert a comprensive force on the lower chord member 58. By virtue of the trussed girder 51, large span contructions have become economically feasible, because of low weight and high rigidity of the structural members.
In the above conventional trussed girder 51, pre-tensed steel cables 62 are arranged along the upper chord members 57 or the lower chord members 58. It has been impossible, due to geometrical considerations, to arrange the steel cable 62 so that it will not cause a twisting force along the central axis of mass of the chord member in the case where the central axis of mass of the chord member is inside the chord member.
Furthermore, the out-of-plane strength is not sufficiently high in the above-mentioned trussed girder 51. If the span is larger than 50-60 m, it is difficult to first construct the whole structure on the ground and then move it into position by crane, because of low buckling resistance. Hitherto, it was necessary to construct the trussed girders in sections and then attach them one to the other in position by using temporary constructions, and the like.
A continuous bending moment does not act on the column in the trussed girder if a steel cable 62 is arranged along the lower chord member 58 (or the upper chord member 57) extending from a fixture at one end of the lower chord member 58 (or the upper chord member 57) to another fixture at the other end of the lower chord member 58 (or the upper chord member 57), as described above. Conversely, there is a large continuous bending moment acting on the foot of the column when the trussed girder 51 receives a wind load or a superimposed load. The base construction is therefore uneconomical.